Strontium for melt oxidation reduction of magnesium and a method for adding stronium to magnesium

ABSTRACT

A method for suppressing oxidation and burning in a molten magnesium alloy which my contain beryllium, comprising covering the exposed surface of the molten alloy in a protective atmosphere and dissolving strontium into the molten alloy is disclosed. This has the benefit of reducing magnesium losses due to oxidation thereby improving efficiency of the process. There is also disclosed a magnesium casting alloy comprising from about 1% to 10% by weight of aluminum, from about 1% to 30% by weight of zinc, from about 0.004% to 0.05 % by weight of strontium and unavoidable impurities, the balance being magnesium. Finally, there is disclosed a beryllium free magnesium die casting which is essentially free of flux inclusions.

FIELD OF THE INVENTION

[0001] The present invention relates generally to methods for lowering oxidation in a magnesium alloy melt. In particular, the present invention relates to the addition of strontium to a magnesium alloy melt in order to lower oxidation.

BACKGROUND OF THE INVENTION

[0002] Given their light weight, high tensile strength and good corrosion resistance, magnesium alloys have gained widespread popularity. In particular, a number of magnesium alloys are particularly well suited to casting and are currently being widely deployed in the automotive and aerospace industries for constructing lightweight and durable components, especially housings for transmissions and the like.

[0003] Open top vessels such as induction furnaces used to melt non-ferrous metals such as magnesium are typically operated with the surface of the molten bath exposed to the ambient atmosphere. Air contact tends to oxidise the melt, thereby causing: loss of metal, loss of alloying additions and formation of slag leading to difficulty in metal processing; shortening of refractory life; and promotion of non-metallic inclusions in the final castings, pickup of unwanted gases in the metals, porosity, and poor metal recovery.

[0004] One solution is to enclose the induction furnace in a vacuum or atmosphere chamber for melting and/or processing of the metals. However, molten magnesium has a high vapour pressure and one drawback of molten alloys comprised of more than 3% by weight of magnesium is that substantial amounts of magnesium are lost to vaporisation. The high vaporisation rate also means that the molten magnesium cannot be held in a vacuum as this would lead to even greater losses. Exposure of molten magnesium to air, however, leads to the formation of magnesium oxide, or magnesia, given magnesium's relatively high affinity to oxygen.

[0005] Oxidation of metals can either be a problem or a means of protection. Although some metals oxidise more readily than others, when exposed to a normal atmosphere all metals will form some surface oxide in a very short time. The key to the behaviour of the metal is whether the oxide that does form creates a continuous, adherent, non-porous protective layer that prevents further oxidation or other corrosive attack on the metal. One measure of this behaviour is the Pilling-Bedworth (P-B) ratio which calculates the ratio of the oxide volume to the volume of the metal it replaces. Although in practice not all elements behave in quite so simple a manner, if the P-B ratio is less than 1 it can be generally said that the oxide is porous and discontinuous and will not protect the metal. Magnesium, with a P-B ratio of about 0.81, forms a loose permeable oxide on the molten metal surface. This allows oxygen to pass through and support burning below the oxide layer at the surface of the molten metal.

[0006] Therefore, when magnesium becomes molten it has a tendency to oxidise and burn unless care is taken to protect the molten surface metal against oxidation. Traditionally, protection of the molten alloy using either a flux sprinkled on the molten metal or a protective gas cover to exclude oxygen has been used. In die casting, however, the use of a flux can lead to operational difficulties, for example flux inclusions in the casting are not uncommon. In some casting applications the use of argon as a protective gas has been proposed but has the drawback that it may lead to explosions. Additionally, the use of argon for sand or gravity casting can be prohibitively expensive due to losses. Due to these drawbacks, the industry has concentrated on the use of a combination of 0.04% to 0.3% sulphur hexafluoride (SF₆) with dry air and in many cases carbon dioxide (CO₂).

[0007] An inorganic fluoride, SF₆ is a non-toxic, inert, insulating and cooling gas of high dielectric strength and thermal stability. At low concentrations of SF₆ in air (<1%), a protective thin film of MgO (and MgF₂) is formed on the magnesium melt surface. However, the use of SF₆ and air has some drawbacks. One major drawback is that SF₆ adds to greenhouses gasses and requires a comparatively long period of time to decompose once released into the atmosphere.

[0008] In any case, although the use of protective gas improves losses to oxidation and burning, other materials must typically be added to the melt before acceptable efficiency can be achieved. In this regard, beryllium is used as an additive in the production of magnesium alloys in the range of 5-15 ppm for its positive effect in improving fluidity during casting and reducing the oxidation which in turn improves yield by reducing losses resulting from oxidation. In order to reduce oxidation, small amounts of beryllium are typically dissolved in the molten magnesium melt. As all oxides form a slag on the surface of the melt which is removed and discarded, reduction of the amount of magnesium oxide formed increases the efficiency of the process by increasing the amount of magnesium alloy available for use.

[0009] However, one drawback with beryllium is its toxicity which is becoming an important health concern. Indeed, inhalation of beryllium dust, fumes and soluble salts can lead to chronic beryllium disease (or Beryllioisis), even at very low levels of exposure. Beryllium disease can lead to irreversible and sometimes fatal scarring of the lungs.

SUMMARY OF THE INVENTION

[0010] The present invention addresses the above and other drawbacks by providing a method for suppressing oxidation and burning in a molten magnesium alloy which may contain traces of beryllium, comprising covering the exposed surface of the molten alloy in a protective atmosphere and dissolving strontium into the molten alloy. In a particular embodiment 0.004% to 0.05% by weight of strontium is dissolved into the molten alloy. In another particular embodiment the strontium is in the form of a strontium-aluminum master alloy comprised of about 70 to 95% by weight strontium which is dissolved in the molten alloy at a temperature of about 670° C. to 700° C. In another particular embodiment pure strontium is dissolved in the molten alloy at a temperature of about 650° C. to 680° C.

[0011] There is also provided a magnesium casting alloy comprising from about 1% to 10% by weight of aluminum, from about 1% to 30% by weight of zinc, from about 0.004% to 0.05% by weight of strontium and unavoidable impurities, the balance being magnesium. In a particular embodiment the magnesium casting alloy is comprised of about 0.007% by weight of strontium; and the balance being the alloy designated AZ91E.

[0012] Finally, there is provided a beryllium free magnesium die casting which is essentially free of flux inclusions. The casting is comprised of from about 1% to 10% by weight of aluminum, from about 1% to 30% by weight of zinc, from about 0.01% to 0.05% by weight of strontium, and unavoidable impurities, the balance being magnesium.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

[0013] Illustrative embodiments according to the present invention will now be described.

[0014] The illustrative embodiments are disclosed in the form of a series of experiments. Two series of experiments were carried out: the first series using a small laboratory size furnace for producing small quantities of alloys; and the second using an industrial size furnace for producing production quantities of alloys.

[0015] The magnesium alloy AZ91, which is used for between 60 and 70% of all magnesium die-castings, provides the basic magnesium alloy used in the experiments. Although AZ91 has been used as the basis for the experiments, it will be understood that the use of strontium as an oxidation reducer may also apply to other magnesium alloys, especially those where beryllium is currently used as an additive for oxidation reduction and in particular those alloys conforming to ASTM 60.

[0016] During production of castings, magnesium alloy is typically melted and held in a molten state for up to 24 hours. During this time additional magnesium alloy ingots may be added to the melt to replenish the supply. In a production facility, the surface of the molten magnesium alloy is typically protected from the ambient atmosphere by a combination of dry-air, and in some cases CO₂, mixed with a small concentration of SF₆.

[0017] It is not understood exactly why additives to a magnesium melt such as beryllium or strontium reduce oxidation. Given that little or none of the strontium dissolved into the melt is lost during the process and continues on as an element in the cast magnesium, it would appear that the strontium is not forming part of a layer on the surface which is later precipitated out as an oxide. However, strontium is a surface active element which likely goes to the surface of the melt and potentially promotes the formation of a magnesium oxide layer which is harder and denser than would otherwise be the case. Alternatively, the addition of strontium may lower the vaporisation rate of the magnesium alloy or affect its P-B ratio. It may also be a combination of these factors.

[0018] Note that the above are speculative and provided for information only. They should not be construed as limiting the scope of the invention.

[0019] Experiment 1: In order to test the comparative effects of strontium as an oxidation reducer when using a protective gas comprised of dry air and sulphur hexafluoride (SF₆) a series of 0.5 kg samples of magnesium alloy were prepared.

[0020] The first sample was prepared by melting a small ingot of Norsk Hydro magnesium product designated AZ91D under a dry-air/0.35% SF₆ protective-gas atmosphere using a 2 kg steel crucible and a Lindberg Blue M electric resistance furnace. AZ91D is designated as a magnesium die casting alloy containing aluminum, zinc and other trace elements with between 5-15 parts-per-million (ppm) by weight of beryllium.

[0021] A second sample was prepared from a small ingot of Magnesium Elektron Limited product designated AZ91E. AZ91E is a magnesium casting alloy containing only trace amounts (<0.001%) of beryllium.

[0022] Third, fourth and fifth samples were prepared also using magnesium Elektron Limited's AZ91E magnesium casting alloy. However, strontium was introduced into the melt in the form of a strontium-aluminum master alloy comprised of 90% strontium and 10% aluminum supplied by Timminco Metals. The amount of strontium-aluminum master alloy added to the melt was in each sample adjusted in order to achieve target sample melts having 40, 75 and 150 ppm strontium. The strontium-aluminum master alloy was added at 680° C. and held for 20-30 minutes for complete dissolution.

[0023] It will be apparent that although for the purposes of the present experiments a strontium-aluminum master alloy was used in order to add strontium to the melt, the master alloy can be replaced by pure strontium. Strontium in its pure form is pyrophoric, and there is a possibility that it will ignite when exposed to air, especially in humid environments. Care must therefore be taken to ensure that the surface of the pure strontium does not come into extended contact with air. This is typically done by coating the surface of the strontium in oil and wrapping it in aluminum foil. The foil/strontium assembly is then added to the molten alloy as required. Alternatively, pure strontium is often available in sealed aluminum containers. Immediately prior to use the container is punctured and the entire assembly added to the molten alloy.

[0024] Each of the samples was subject to analysis in order to determine the exact make-up of the alloys. The results are table following: TABLE I ALLOY Al % Be % Mn % Zn % Si % Cu % Fe % Ni % Sr % Ca % AZ91D 8.55   0.0009 0.23 0.72 0.0246 0.0002 0.0006 0.0006 N/D N/D AZ91E 8.80 <0.0002 0.28 0.59 0.0022 <0.0002   <0.0010   0.0002 <0.0002   N/D AZ91E + 40 ppm 8.58 <0.0001 0.27 0.59 0.0105 0.0009 0.0022 0.0004 0.0038 N/D Sr AZ91E + 75 ppm 8.45 <0.0001 0.26 0.58 0.0097 0.0009 0.0021 0.0004 0.0076 N/D Sr AZ91E + 150 ppm 8.56 <0.0001 0.27 0.56 0.0086 0.0007 0.0018 <0.0002   0.0137 N/D Sr

[0025] For all five samples, after complete melting of the alloy ingots and dissolution of the strontium-aluminum master alloy, the melt surface was skimmed using a steel ladle. The melt was then held at 680-682° C. for 5 hours. At the end of that time the melt surface was carefully skimmed again and the amount of melt oxide that was skimmed weighed in order to determine the amount of oxidation. The results are tabled following: TABLE II Weight of Oxides/ Actual charge skimmed oxides Charge Ratio ALLOY (g) (g) (%) AZ91D 549.76 11.016 2.00% 550.17 12.581 2.28% 549.76 9.624 1.75% Average AZ91D 549.90 11.07 2.00% AZ91E 550.50 34.821 6.32% 550.01 49.600 9.01% 550.62 14.477 2.63% Average AZ91E 550.38 32.966 6.00% AZ91E + 40 ppm Sr 547.29 12.79 2.33% AZ91E + 75 ppm Sr 549.56 5.74 1.04% AZ91E + 140 ppm Sr 545.37 9.31 1.70%

[0026] The table below gives values for the degree of oxidation of each of the sampled alloys normalised against the alloy designated AZ91E. TABLE III AZ91E 1.00 AZ91D (with 9 ppm Be) 0.33 AZ91E with 40 ppm Sr 0.39 AZ91E with 75 ppm Sr 0.17 AZ91E with 140 ppm Sr 0.28

[0027] As is evident from the above, the addition of 75 ppm strontium provided an extremely efficient replacement for beryllium.

[0028] Experiment 2: In order to test the comparative effects of strontium as an oxidation reducer when using a protective gas comprised of air, CO₂ and SF₆ in and industrial setting, experiments were carried out using a Dynarad electric resistance furnace having a 75 kg capacity, an Air/CO₂/SF₆ cover gas and an ingot mould assembly.

[0029] AZ91E alloy having no beryllium provided the base magnesium alloy for use in the second series of experiments. Experiments involved melting the alloys, skimming the surface and holding for 24 hours at the end of which the melt was skimmed again and the oxide removed into an ingot mould. The oxide was then weighed to determine the degree of oxidation in the alloy melt. The initial charge of AZ91E was 75.51 kg.

[0030] The control experiment involved the melting and holding of beryllium free AZ91E melt. However, due to excessive, uncontrollable oxidation and burning the melt could only be held for three hours and therefore no oxide weight at the end of 24 hours could be obtained. Oxidation at the end of three hours had already reached 7.3 kg or 2.4 kg/hour.

[0031] Similar to Experiment 1, the strontium containing melts were prepared by melting AZ91E alloy and adding the strontium as a strontium-aluminum master alloy comprised of 90% strontium and 10% aluminum alloy. Three test melts were prepared having target strontium levels of 100, 250, and 400 ppm. The melt was stirred, and after allowing 45 minutes for strontium dissolution, the melt was sampled and analysed. The melt was also sampled and analysed after 24 hours holding and the composition determined. The results of the analysis of the composition of the melts are tabled following: TABLE IV Alloy Al Mn Zn Si Cu Fe Ni Be Sr Ca AZ91E 8.80 0.28 0.59 0.0022 <0.0002   <0.0010     0.0002 <0.0002  <0.0002   <0.0010 AZ91E + 100 ppm 9.32 0.21 0.46 0.0185 0.0007 0.0023 <0.0010 <0.00005 0.0130 <0.0002 Sr before holding AZ91E + 100 ppm 9.58 0.19 0.46 0.0212 0.0008 0.0043 <0.0009 <0.00005 0.0113 <0.0002 Sr after 24 hours AZ91E + 250 ppm 9.11 0.21 0.45 0.0237 0.0009 0.0064 <0.0009 <0.00005 0.0248 <0.0002 Sr before holding AZ91E + 250 ppm 8.69 0.21 0.40 0.0189 0.0023 0.0082 <0.0009 <0.00005 0.0235 <0.0002 Sr after 24 hours AZ91E + 400 ppm 8.63 0.21 0.42 0.0299 0.0011 0.0084 <0.0009 <0.00005 0.0439 <0.0002 Sr before holding AZ91E + 400 ppm 8.75 0.21 0.41 0.0364 0.0010 0.0082 <0.0009 <0.00005 0.0389 <0.0002 Sr after 24 hours

[0032] The average actual levels of strontium in the melts over 24 hours were as follows: TABLE V AZ91E  <2 ppm strontium AZ91E with 100 ppm Sr 122 ppm strontium AZ91E with 250 ppm Sr 242 ppm strontium AZ91E with 400 ppm Sr 414 ppm strontium

[0033] Each melt surface was then skimmed and held for 24 hours at the end of which the surface oxide was removed to be weighed. The results are summarised in the following table. TABLE VI Actual Weight of Alloy Target Holding time Average Sr Skimmed Weight of Oxide/Charge composition (Hours) content Oxides (kg) Oxide/Hour Ratio (%) Burning AZ91E  3  <2 ppm 58 (estimated)  2.4 kg/hr 77.00%  Uncontrolled burning AZ91E + 100 ppm 24 122 ppm 3.042 0.125 kg/hr 5.00% Very low Sr AZ91E + 250 ppm 24 242 ppm 2.365  0.1 kg/hr 3.00% No burning Sr AZ91E + 400 ppm 24 414 ppm 3.760  0.16 kg/hr 5.00% Very low Sr

[0034] Results indicated that with an actual initial addition of only 130 ppm of strontium, the AZ91E oxidation and burning was reduced and the melt could be held for 24 hours. The melt surface was protected with a rather thick oxide layer but was quite stable and did not burn. The weight of the oxide removed from the surface of melt comprised of AZ91E and initially 130 ppm strontium after 24 hours was 3 kg or 0.125 kg/hr. Oxidation was further reduced in the AZ91E+240 ppm strontium melt and the oxide weight at the end of 24 hours was 2.4 kg or 0.1 kg/hr. The AZ91E+414 ppm strontium melt shows an oxidation rate and degree similar to the AZ91E+120 ppm strontium melt.

[0035] It can be concluded that trace additions of strontium are effective in suppressing the oxidation in AZ91E alloy with no beryllium or trace amounts of beryllium. The level of strontium to be added may depend on the exposed melt surface hence higher strontium levels than those determined in small scale experiments may be required for more effective protection. Indeed, surface oxidation and the formation of an oxide layer may be adversely affected by the turbulence in the melt, disturbance of the surface skin or other industrial conditions.

[0036] As noted about 240 ppm strontium highly is efficient in suppressing oxidation and burning in magnesium alloys. The table below gives values for the degree of oxidation in 24 hour as normalised against AZ91E. TABLE VII AZ91E 1.00 AZ91E + 122 ppm Sr 0.07 AZ91E + 242 ppm Sr 0.04 AZ91E + 414 ppm Sr 0.07

[0037] Although the present invention has been described hereinabove by way of a preferred embodiment thereof, this embodiment can be modified at will, within the scope of the appended claims, without departing from the spirit and nature of the subject invention. 

What is claimed is:
 1. A method for suppressing oxidation and burning in a molten magnesium alloy, comprising: covering the exposed surface of the molten alloy in a protective atmosphere; and dissolving strontium into the molten alloy.
 2. A method for suppressing oxidation and burning in a molten magnesium alloy containing only traces of beryllium, comprising: covering the exposed surface of the molten alloy in a protective atmosphere; and dissolving strontium into the molten alloy.
 3. A method for suppressing oxidation and burning in a molten magnesium alloy as in claims 1 and 2 wherein the protective gas includes between about 0.04% and 0.35% sulphur hexafluoride and dry-air.
 4. A method for suppressing oxidation and burning in a molten magnesium alloy as in claim 3 wherein the protective gas further includes carbon dioxide.
 5. A method for suppressing oxidation and burning in a molten magnesium alloy as in claims 1, 2, 3 and 4 wherein about 0.004% to.0.05% by weight of strontium is dissolved into the molten alloy.
 6. A method for suppressing oxidation and burning in a molten magnesium alloy as in claims 1, 2, 3, 4 and 5 wherein the strontium is in the form of a strontium-aluminum master alloy comprised of about 70 to 95% by weight strontium which is dissolved in the molten alloy at a temperature of about 670° C. to 700° C.
 7. A method for suppressing oxidation and burning in a molten magnesium alloy as in claims 1, 2, 3, 4 and 5 wherein pure strontium is dissolved in the molten alloy at a temperature of about 650° C. to 680° C.
 8. A magnesium casting alloy comprising: from about 1% to 10% by weight of aluminum; from about 1.5% to 30% by weight of zinc; from about 0.004% to 0.05% by weight of strontium; and unavoidable impurities; the balance being magnesium.
 9. A magnesium casting alloy comprising: from about 9% to 10% by weight of aluminum; from about 1% to 2% by weight of zinc; from about 0.004% to 0.05% by weight of strontium; and unavoidable impurities; the balance being magnesium.
 10. A magnesium casting alloy comprising: from about 9% to 10% by weight of aluminum; from about 1% to 2% by weight of zinc; about 0.007% to 0.0250% by weight of strontium; and unavoidable impurities; the balance being magnesium.
 11. A magnesium casting alloy comprising: from about 0.007% by weight of strontium; and the balance being the alloy designated AZ91E.
 12. A magnesium die casting being essentially free of flux inclusions comprising: from about 1% to 10% by weight of aluminum; from about 1% to 30% by weight of zinc; from about 0.01% to 0.05% by weight of strontium; and unavoidable impurities; the balance magnesium. 